Rewritable and removable optical disks have been widely used as file memories for personal computers. Hitherto, various technologies for increasing the recording density of the optical disks have been developed.
Methods for recording marks (mark position recording/mark length recording), methods for dividing regions into sectors (Constant Angular Velocity/Zone Constant Angular Velocity: CAV/ZCAV), and the like are known as methods for increasing the recording density. Other than these methods, a method for reducing the track pitch or the mark length by using light having a shorter wavelength is known. This method has significantly contributed to the increase of recording density.
Hitherto, a so-called land recording method has been used, in which data are only written to and read from tracks, each track being formed with a land disposed between guide grooves (pregrooves). Recently, however, a so-called land/groove recording method has been also put into practical use for optical disks, in which the data are written in the grooves in addition to the lands, with the width of each groove (width in the direction of the shorter axis of the groove) being increased. In the present description, regions of an optical disk where data can be written to and read from are referred to as rewritable regions.
An optical disk must be recorded in advance with read-only data which are so-called control data, such as the type of the optical disk and recording conditions, and which are not erasable. An example of the layout of the control data on an optical disk is shown in FIG. 8. In FIG. 8, numeral 21 denotes an optical disk, numeral 22 denotes a rewritable region, and numeral 23 denotes a region in which the control data are recorded (hereinafter referred to as a control data region).
In the land recording method, the control data are recorded on the optical disk with concavo-convex pit rows being formed along land tracks. In a land/groove recording method, the control data are recorded with the concavo-convex pit rows being formed in a flat region instead of the rewritable region. In this case, the capacity of the optical disk is inefficiently used by providing the flat region.
Another method for recording the control data is disclosed in, for example, Japanese Unexamined Patent Publication No. HEI 9(1997)-274733, in which recording is performed by forming pit rows in a flat region, the pit rows having the same track pitch as grooves.
However, since the concavo-convex pits are formed in a flat surface which has no grooves, an optical head is likely to mistakenly detect the track pitch when the optical head seeks the flat surface, and there is a risk in that desired control can not be performed.
In addition to the control data, the optical disk has read-only data recorded, which are header data, such as a sector mark, a synchronizing signal (VFO), an address mark, and ID (IDentification) data, at the header of each sector as a unit which forms the rewritable region, the recording being performed by forming the concavo-convex pit rows, so that high-speed access is possible. In particular, the header data are recorded in the form shown in FIG. 8. In FIG. 8, numeral 25 denotes a sector, numeral 26 denotes a region in which the header data are recorded (hereinafter referred to as a header data region), and numeral 27 denotes a rewritable region. The header data region 26 includes a sector-mark region 28, a VFO region 29, an address-mark region 30, an ID-data region 31, and the like in the order in the track direction.
In a magneto-optical disk, which is one type of optical disk, a technology for high-resolution reading from magneto-optical marks each having a size smaller than the diffraction limit of the reading light (magnetic super resolution technology) is used. However, since the resolution for reading the above-described concavo-convex pits is not increased by this technology, data which are written by using the concavo-convex pits having the same pitch as the magneto-optical marks along the circumference of the disk (in the track direction) could not be read at high resolution. Therefore, in order to read data from the magneto-optical marks and data (the control data and header data) from the concavo-convex pits, it is necessary to make the recording frequency of the latter lower than that of the former.
It is also difficult to read in the radial direction (the seek direction) of a magneto-optical disk from the concavo-convex pits formed at the same track pitch as that of the magneto-optical marks, at high resolution while reducing crosstalk. Therefore, methods in which the concavo-convex pits for header data in land tracks and in groove tracks are offset from each other along the circumference have been proposed (for example, in Japanese Unexamined Patent Publication Nos. HEI 6(1994)-28729 and HEI 10(1998)-79125).
Also, a method has been proposed (for example, in Japanese Unexamined Patent Publication No. HEI 7(1995)-110944), in which, since a sector mark, for example, of the header data is commonly used in each track, the sector-mark regions, each indicating the start of a header data region, are disposed every two tracks, and when the optical head is positioned at a track where no sector mark is provided, the optical head detects crosstalk data from the sector-mark regions of the adjacent tracks as the sector mark of the track at which the optical head is positioned (for example, in Japanese Unexamined Patent Publication No. HEI 7(1995)-110944). However, in this method, there is a problem in that the length of the sector mark which is read from the crosstalk is recognized to be smaller than that of the sector mark which is read by the optical head when it is positioned at the track on which the sector mark is provided.
Another method has been proposed (for example, in Japanese Unexamined Patent Publication No. HEI 9(1997)-81965), in which the concavo-convex pits for sector marks are disposed in the land tracks and in the groove tracks. However, a problem has been found in this method in that the seeking performance of the optical head is reduced because large distortion of a track error signal is produced.